Re: [Qemu-devel] [PATCH v2 1/5] virtio-net: use the backend cross-endian capabilities

[Cornelia Huck]

On Mon, 11 Jan 2016 17:12:21 +0100
Greg Kurz <address@hidden> wrote:

> When running a fully emulated device in cross-endian conditions, including
> a virtio 1.0 device offered to a big endian guest, we need to fix the vnet
> headers. This is currently handled by the virtio_net_hdr_swap() function
> in the core virtio-net code but it should actually be handled by the net
> backend.
> 
> With this patch, virtio-net now tries to configure the backend to do the
> endian fixing when the device starts (i.e. drivers sets the CONFIG_OK bit).
> If the backend cannot support the requested endiannes, we have to fallback
> onto virtio_net_hdr_swap(): this is recorded in the needs_vnet_hdr_swap flag,
> to be used in the TX and RX paths.
> 
> Note that we reset the backend to the default behaviour (guest native
> endianness) when the device stops (i.e. device status had CONFIG_OK bit and
> driver unsets it). This is needed, with the linux tap backend at least,
> otherwise the guest may loose network connectivity if rebooted into a

s/loose/lose/

> different endianness.
> 
> The current vhost-net code also tries to configure net backends. This will
> be no more needed and will be reverted in a subsequent patch.
> 
> Signed-off-by: Greg Kurz <address@hidden>

(...)

> +static void virtio_net_vnet_status(VirtIONet *n, uint8_t status)
> +{
> +    VirtIODevice *vdev = VIRTIO_DEVICE(n);
> +    NetClientState *peer = qemu_get_queue(n->nic)->peer;
> +
> +    if (virtio_net_started(n, status)) {
> +        int r;
> +
> +        /* Before using the device, we tell the network backend about the
> +         * endianness to use when parsing vnet headers. If the backend can't
> +         * do it, we fallback onto fixing the headers in the core virtio-net
> +         * code.
> +         */
> +        if (virtio_is_big_endian(vdev)) {
> +            r = qemu_set_vnet_be(peer, true);
> +        } else {
> +            r = qemu_set_vnet_le(peer, true);
> +        }
> +
> +        n->needs_vnet_hdr_swap = !!r;
> +    } else if (virtio_net_started(n, vdev->status)) {
> +        /* After using the device, we need to reset the network backend to
> +         * the default (guest native endianness), otherwise the guest may
> +         * loose network connectivity if it is rebooted into a different

here again

> +         * endianness.
> +         */
> +        if (virtio_is_big_endian(vdev)) {
> +            qemu_set_vnet_be(peer, false);
> +        } else {
> +            qemu_set_vnet_le(peer, false);
> +        }
> +    }
> +}
> +

Reviewed-by: Cornelia Huck <address@hidden>